Prior fluid sensing devices have been proposed in which a pair of spaced-apart electrodes are supported on a surface layer. One type of such device for sensing humidity utilizes a hygroscopic substance, such as an ionic salt, which is applied to the surface between two conductive electrodes, but this type of sensor is easily damaged and is subject to contamination.
Another related type of prior fluid sensing device relies on the absorbed fluid on the surface between the electrodes. This type of device is unacceptable for many applications because it requires special measuring circuits due to the high surface resistivity of the insulating substrate.
The construction of a fluid sensor with two electrodes applied on a supporting surface is desirable, however, because of the resulting simplicity and low profile which is provided and the fact that the flat surface area of the substrate provides a large area for adsorption which increases the sensitivity of the device.
It is, therefore, the object of the present invention to provide a fluid sensing device in which a resistance of a predetermined value may be applied on a molecular sieve layer between two electrodes so as to make electrical contact therebetween.
It is a further object of the present invention to provide a fluid sensor in which a resistance path is formed on a molecular sieve substrate in a manner such that desirable properties of the layer may be utilized to advantage.
It is an additional object of the present invention to provide a fluid sensor that is made up of a molecular sieve layer, two spaced-apart electrodes and a resistance strip deposited on the molecular sieve layer between the two electrodes, and preferably also a heating resistor which is deposited on the molecular sieve layer, or is positioned in proximity thereto.